In many antenna applications, for example such as for use with aircraft and vehicles, an antenna with a broad bandwidth is required. For such applications, the so-called "frequency-independent antenna" ("FI antenna") commonly has been employed. See for example, V. H. Rumsey, Frequency Independent Antennas, Academic Press, New York, N.Y., 1966. Such frequency-independent antennas typically have a radiating or driven element with spiral, or log-periodic structure that enables the frequency-independent antenna to transmit and receive signals over a wide band of frequencies, typically on the order of a 9:1 ratio or more (a bandwidth of 900%). For example, European Patent Application No. 86301175.5 of R. H. DuHamel entitled "Dual Polarized Sinuous Antennas", published Oct. 22, 1986, publication No. 0198578 (See also U.S. Pat. No. 4,658,262 dated Apr. 14, 1987), discloses frequency-independent antennas with a log-periodic structure called "sinuous."
In a conventional frequency-independent antenna, a lossy cylindrical cavity is positioned to one side of the antenna element so that when transmitting, energy effectively is radiated outwardly from the antenna only from one side of the antenna element (the energy radiating from the other side of the antenna element being dissipated in the cavity). However, high-performance aircraft, and other applications as well, require that the antenna be mounted substantially flush with its exterior surface, in this case the skin of the aircraft. This undesirably requires that the cavity portion of the frequency-independent antenna be mounted within the structure of the aircraft, necessitating that a substantial hole be formed therein to accommodate the cylindrical cavity, which typically is at least two inches deep and several inches in diameter. Also, the use of a lossy cavity to dissipate radiation causes about half of the radiated power to be lost, requiring a greater power input to effect a given level of power radiated outwardly from the frequency-independent antenna.
In recent years the so-called "microstrip patch antenna" has been developed. See for example, U.S. Pat. No. Re. 29,911 of Munson (a reissue of U.S. Pat. No. 3,921,177) and U.S. Pat. No. Re. 29,296 of Krutsinger, et al. (a reissue of U.S. Pat. No. 3,810,183). In a typical microstrip patch antenna, a thin metal patch, usually of circular or rectangular shape, is placed adjacent to a ground plane and is spaced a small distance therefrom by a dielectric spacer. Microstrip patch antennas have generally suffered from having a narrow useful bandwidth, typically less than 10%.
Co-pending U.S. patent application Ser. No. 07/695,686 recites a multi-octave spiral-mode microstrip antenna which overcomes many of the prior art limitations. This spiral-mode antenna approaches the bandwidth of frequency-independent antennas and is nearly flushly mounted above a ground plane. However, multi-mode operation of a spiral-mode microstrip antenna requires the spiral to be of circumference at least m.lambda., where m is the highest desired mode and .lambda.is the wavelength. Thus, the spiral diameter can become undesirably large, especially at lower frequencies.
Microstrip patch array antennas have also been known in the art. See, for example, Munson, R. E., Conformal Microstrip Antennas and Microstrip Phased Arrays, IEEE Transactions on Antennas and Propogation, p. 74 (January 1974). The Munson article discusses an array of rectangular elements. However, known microstrip arrays, including the Munson design, generally are electrically large (i.e., the antenna is relatively large in comparison with the wavelength of the operating frequency), having individual elements of approximately one-half wavelength in diameter and spaced from one another a distance slightly greater than their diameters.
U.S. Pat. No. 4,766,444 of Conroy et al relates to a conformal "cavity-less" antenna having an array of single-arm spiral elements driven in unison and which are aligned linearly along an outwardly-curved surface. A lossy hex-cell structure spaces the spiral elements away from the ground plane and takes the place of the typical cavity. The resulting antenna is disclosed as being suited for use as an interferometer and tends to suffer from having a narrow useful bandwidth. Again this is an electrically large array.
Accordingly, it can be seen that a need yet remains for an antenna which has a low profile, has a broad bandwidth relative to prior antennas, and is small in physical size. It is the provision of such an antenna that the present invention is primarily directed.